Keep practicing, and you might become an expert. Or maybe you won't. Who knows? Not the experts, suggests a raging debate.

Made famous by Outliers: The Story of Success, by Malcolm Gladwell, the 2008 book's "10,000-hour rule"—the number of hours of practice needed to acquire mastery of a skill—looks increasingly beleaguered.

Underlying arguments over whether winners are made or born, or over nature versus nurture, the disagreement points to deep uncertainty about who should receive expert instruction and how best to teach people to excel.

"No one disputes that practice is important," says psychologist David Zachary Hambrick of Michigan State University in East Lansing. "Through practice, people get better. The question is whether that is all there is to it."

Kicking off the fracas, a study published last May in the journal Intelligence by Hambrick and colleagues suggested that practice explains only about a third of success among musician and chess masters. A series of responses to the study have since appeared in the journal, which is generally seen as a bastion of researchers who argue over the meaning of I.Q. scores.

The debate climaxed last month in the journal with the original study's authors' reply to a reply from psychologist K. Anders Ericsson of Florida State University in Tallahassee. Ericsson is best known for the research on premiere violinists touted in Gladwell's book, which credited the rule to Ericsson and other previous research efforts.

Gladwell summed up the idea behind his deliberate practice rule as, given enough motivation, "if you are smart enough to get through college, you can be a good cardiac surgeon."

Fiddling Around

In last year's Intelligence study, Hambrick's team looked again at case studies of master musicians and chess players, the subjects of Ericsson's research. After quizzing the players on their lifetime hours of deliberate practice (as opposed to performances or play), they concluded that practice accounted for only 30 percent of success in music and 34 percent in chess.

They also found wide variability in the hours of practice. Chess grand masters had put in from 832 to 24,284 hours of work, although the average was around 10,530 hours. Musicians' efforts ranged from 10,000 to 30,000 hours.

The variability in hours of practice washes away any meaning from the 10,000-hour rule, Hambrick suggests.

In his response, Ericsson says that this kind of critique inappropriately mixes data about less-skilled folks into the analysis. (He also notes that he wrote about this kind of variability in practice in a 2013 British Journal of Sports Medicine editorial: "There is nothing magical about 10,000 hours.")

Hambrick counters that Ericsson's reliance on only a few supreme performers for his studies of expertise turns the studies into anecdotes. "If we don't have enough data points in the study to say anything statistically, then it isn't science."

To some extent, says psychologist Phillip Ackerman of Georgia Tech in Atlanta, expertise studies are butting up against the limitations of social science research, where the study variables—human beings—are more squirrelly than are the uniformly behaving atomic particles or molecules of the physical sciences.

Haunting the 10,000-hour rule argument are the specters of two influential, but discredited, scientists.

The first, Francis Galton, the 19th-century father of eugenics, argued that heredity essentially explained all talent and expertise. Galton's racist ideas inspired the Nazi's horrific notions of racial purity.

The second, John Watson, the early-20th-century father of behaviorism, argued,

"Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select—doctor, lawyer, artist, merchant-chief and, yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors."

To his credit, Watson acknowledged he was "going beyond the facts" in this contention. But his 1920s advice on child-rearing led some to treat their children with emotional detachment, which was terrible advice for parents.

Today's 10,000-hour rule debate is often pitched as a fight over which of these extremes, nature or nurture, is more important when it comes to acquiring expertise. And the participants in the debate often paint their opponents as taking an extreme view of how expertise is acquired.

Gladwell, for example, said at the MIT debate that Ericsson is guilty of "expanding and stretching" the 10,000-hour rule toward being an absolute, far from his own view of it as more of a principle for people prescreened for aptitude for a skill. (Last year, he wrote in The New Yorker that complaints about his book "makes me think that there is another Malcolm Gladwell out there, with far more eccentric views than mine.")

Ericsson notes that Gladwell's book mistakes the average of 10,000 hours that experts took to master a skill described in his research with the total they required. "In fact, by the age of twenty, the elite performers had each totaled 10,000 hours of practice," says Gladwell in Outliers.

Aside from obvious genes for height or body size that help people in sports such as basketball, Ericsson says his real position is simply that he doesn't see evidence for genes that help people acquire expert levels of performance, as Epstein's book on elite athletes describes.

"The genes might well be there, but until we definitely identify them, I think we can't go beyond that," Ericsson says. In the era of the human genome, he argues against a stampede toward genetic explanations for things.

Hambrick and other 10,000-hour critics see this as "moving the goalposts" in a scientific debate, backpedaling from an earlier strong position to a weaker one while refusing to concede.

Plenty of studies suggest that aside from practice hours, individual differences help explain success, Georgia Tech's Ackerman says in Intelligence. Such differences range from socioeconomics to coaching to I.Q.

The big difference between them was the amount of good coaching they received at a young age.

Think of the Children

The participants in the disagreement often voice two opposing concerns.

One fear Ericsson expresses is that if talent is viewed as somehow innate and not the result of practice, disadvantaged kids will be cut off from opportunities in education and sports.

"Nobody is going to be able to fly," Ericsson says. "But some kids are going to be blown away by the number of things they can do if they're given the chance."

On the other hand, Ackerman worries that telling people they just need to practice more might set them up for failure: "The odds are pretty good, but not impossible, that if you have an I.Q. of 70, you're probably not going to get a Ph.D. in particle physics."

Practice, then, is just one more ingredient in success. "There aren't innate abilities. Aside from our reflexes, everything is learned in one way or the other," Ackerman says. "Motivation, timing, and luck are all in there too."

Correction: The original story incorrectly attributed author David Epstein's first name.